Method of producing sintered ceramic material

ABSTRACT

In a method of manufacturing a sintered ceramic material using the heat generated in a thermit reaction as a heating source, a pre-heating is applied preceding to the sintering step or a mixture comprising: (A) at least one ceramic powder, (B) at least one non-metallic powder selected from the group consisting of carbon, boron and silicon, and (C) a metal powder and/or a non-metallic powder other than the above-mentioned (B) is used. Homogeneous and dense sintered ceramic material or sintered composite ceramic material can be obtained by this method, and the fine texture thereof, and the phase constitution, the phase distribution and the like of the composite ceramic phase can be controlled sufficiently.

TECHNICAL FIELD OF THE INVENTION

The present invention concerns a novel method of manufacturing asintered ceramic material and, more in particular, it relates to amethod of manufacturing a sintered ceramic material capable of modifyingvarious characteristics of a sintered ceramic material over a wide rangeeven for a raw material of an identical composition and excellent in thecontrollability and the homogenity of a fine texture by sufficientlycontrolling the fine texture in a sintering step for a short period oftime depending on the purpose.

The present invention also relates to a method of manufacturing a novelsintered composite ceramic material and, more in particular, it relatesto a method of manufacturing a sintered composite ceramic material ofextremely good quality in which the phase composition of a compositeceramic, the structure and the distribution of various kinds ofconstituent phases and the crystal grain size are controlledsufficiently, by conveniently proceeding a thermit reaction under apressurized state, and utilizing a great amount of heat of reactionobtained in a short period of time.

BACKGROUND OF THE INVENTION

Various characteristics of a sintered ceramic material depends stronglyon a fine texture thereof. Accordingly, ceramic sintering methods havebeen developed, selected and improved as various kinds of heatprocessing techniques so as to find a method of controlling the finetexture required for attaining aimed characteristics of a sinteredceramic material regarding various factors such as the kind of the rawceramic powder, characteristics of the material powder, absence orpresence of a sintering binder or material of ceramic in the pre-stageof sintering, for example, powder charging ratio of a molding product,crystal structure and physical properties of powder. One of the greatestsubjects on the method of manufacturing such a sintered ceramic materialis to establish a sintering method having controllability of a finetexture over a wide range capable of attaining aimed characteristics ofthe sintered ceramic material while ensuring high densification.

Means for dissolving the subject are generally classified into (i)development of a raw sintering powder having high quality and versatilenature, (ii) development of a powder processing technique including, forexample, development of various kind of powder molding techniques whiletaking the performance and the productivity of the sintered materialalso into consideration, (iii) search for the fine texture controllingagent such as various kinds of binders and (iv) development forsintering and processing technique therefor including, for example,development of a heating method or heat treatment process, for whichvarious research and development have been conducted. However, in anactual sintering process, since the existent powder processing techniqueand the heat sintering process technology are in an extremely complicaterelationship the fine texture greatly varies locally or entirely oroften lacks in the reproducibility, also in the sintering of a ceramicpowder of constant powder characteristics and molding conditions, due toslight fluctuations of parameters in the sintering process (temperature,heating rate, etc.) or or slight changes such as of inevitableimpurities in the powder and mold density.

On the other hand, for the manufacture of a sintered composite ceramicmaterial, a method of sintering and densification by heating for a longperiod of time has been adopted usually. One of most prominent subjectsin the conventional manufacturing method is that it is not possible toobtain a polyphase sintered ceramic material which is dense andcomprises fine crystal grains and in which various kind of ceramic phaseconstitutions in the sintering material is sufficiently controlled inaccordance with the purpose. For promoting the densification, there is apress-sintering method, for example, a hot press (HP) or hot isostaticpress (HIP) method and, further a high pressure sintering method ofapplying heating under a higher pressure is also effective. On the otherhand, for promoting the densification in a vacuum sintering method or anatmospheric pressure sintering method, various kinds of aids are used toattain the densification by a sort of polyphase composition. However, itis still difficult even by such a means to obtain a polyphase sinteredceramic material in which various kind of ceramic phase constitutions inthe sintered material is sufficiently controlled and there has been thefollowing problems.

(1) Crystal grains are grown coarser during the densification tending toleave pores. Further, the coarser growth of the crystal grains formsthermal stresses in the sintered material tending to cause destruction,particularly, in highly anisotropic ceramic.

(2) While addition of aids such as a grain size growth inhibitor isindispensable for preventing crystal grains from growing coarser duringsintering under densification, the aids often reduce the characteristicsof the sintered material. Further, even in a case of sufficientlymodifying raw sintering material into a fine powder and preventing thecrystal grain growth by low temperature sintering, it takes a long timefor the densification and, also, control for the sintering temperature,time and atmosphere are extremely difficult.

(3) Although the crystal grains can be kept from growing coarser in thesintered material by the means (2) described above, the shape of each ofthe ceramic phases become indistinct or the attainment of the aimedcomposite phase constitution is remarkably restricted.

Lack of the controllability for the fine texture caused by thecomplicate relationship between the powder processing technique and theheat sintering processing technique involves the following problems.

(a) Conventional sintering methods represented, for example, by anatmospheric sintering or vacuum sintering requires a long time forheating and densification the material to be sintered and various kindof sintering mechanisms proceed stepwise or simultaneously in the courseof the process. Accordingly, control for the fine texture is verydifficult and the fine texture is determined unimeaningly depending onthe conditions for attaining the densification of the sintering material(for example, temperature and time). As a result, the controllabilityfor the fine texture is remarkably reduced.

(b) For the promotion and acceleration of the densification,press-sintering method under application of pressure can be adopted.However, in any of hot press (HP), hot isostatic press (HIP) and highpressure sintering method usually employed, since the heating method issimilar to indirect heating like that in (a) above, provides relativelya slow heating rate and low controllability, application of the pressureis useful for removal of pores in the sintered ceramic material but doesnot function sufficiently for the control of the fine texture in thesintered material.

(c) For controlling the heating rate to the powder material to besintered described in (a) and (b) above, it is considered to apply acurrent supply sintering method in which electric current is directlysupplied to the powder to be sintered and the ohmic heat generation isutilized for heating. For instance, as a method of manufacturing acermet or a conductive composite ceramic, a current supply hot presssintering method is utilized for shortening the time required fordensification (Powder and Powder Metallurgy, vol. 32, No. 6, p 215-218).However, such a method is effective only for the sintering of conductiveceramic or a mixture of a conductive ceramic with a semiconductive orinsulative ceramic of a particular composition and can not be applied tothe sintering of semiconductive or insulative ceramic. In addition,there is also a drawback that the temperature in the ohmic heatgeneration varies depending on local fluctuations of the molding densityof the sintered powder or the locallized variation of the electricresistance of the powder, thereby causing variation of the temperaturedistribution in the sintered material and it is difficult to obtain ahomogeneous fine texture. In addition, since the electric resistance ofthe material to be sintered is usually low, the direct current supplyheating for the conductive material to be sintered has many difficultiesalso from an industrial point of view, for example, installation of agreat current supply system is indispensable.

(d) Various kinds of sintering binders or sintering aids have beendeveloped as a controlling agent for the fine texture, but the effect ofsuch additives on the sintering mechanism and anticipation for theeffect obtained as the result on the characteristics of the sinteredmaterial are still insufficient and they are extremely groping.Accordingly, it is difficult to attain a development of new ceramicmaterial having a controlled fine texture under the consideration ofvarious kind of characteristics of the sintered material, unless variousresults of experiment are obtained and, in addition, there is a notattainable subject not yet overcome for the control of the fine texturein the heat-sintering process as shown in (a), (b) and (c) even when theabove-mentioned binders are used.

(e) Although the powder processing technology has been progressedremarkably in recent years, it is extremely difficult, for example, tomanufacture a ceramic powder of homogeneous grain size distribution andwith no aggregation for the raw material powder in order to obtain ahomogenous fine texture and the manufacturing cost is much expensive. Inaddition, although the technique for uniformly molding a raw materialpowder, particularly, fine raw material powder, having uniform powdercharacteristics has been developed partially, for example, as a hotmolding technique, it has not yet been completed, and a great care isrequired for the control of the fine texture, that is, characteristicsof the sintered material in the powder process and the heat-sinteringprocess, and there is a difficulty that the characteristics of thesintering material varies greatly depending on slight fluctuations andvariations even, if any, of process parameters.

As an attempt for dissolving the foregoing problems, there have beendeveloped various kind of plasma sintering methods (Proceedings of theFirst International Symposium on Ceramic Components for Engine, 1983, p710-715) and microwave sintering methods (Ceramic Bulletin, Vol. 68, No.2, 1989, p 376-386) as another short time sintering method for thepromotion of densification and control of the fine texture shown in theproblems (a)-(c) described above. The plasma sintering method utilizessuper high temperature possessed in plasmas for the sintering under theappropriate control of an atmosphere, which is a process capable ofrapidly heating a material to be sintered at an extremely high energyefficiency and it has been reported that densification and thesuppression for the grain growth during sintering were attainedsimultaneously mainly in oxide series ceramics. On the other hand, themicrowave sintering method is a cold process using microwaves as aheating means, and it has a feature of generating heat at the inside ofthe material to be sintered. As a result, it has been reported thatrapid and uniform heating was possible irrespective of the size ofspecimens and the homogenity of the fine texture of the sinteredmaterial can be improved.

As another short time sintering method combined with a pressuretechnology, a simultaneous synthesis and sintering method of ceramicreferred to as High-Pressure, Self-Combustion Sintering for Ceramics hasbeen developed by using a so-called SHS (Self-Propagating HighTemperature Synthesis) which was studied since 1967 in USSR (refer toJapanese Patent Publication Sho 60-246270 and Comm. Am. Ceram. Soc.,c-224-5, 1984, Nov.). The SHS method is a method of self-heating byusing a burnable exothermic reaction mixture such as a thermitcomposition. For example, a ceramic material can be synthesized from amixture of constituent elements for the ceramic material by using thismethod without external heating by using heat generating compoundforming reaction between each of the elements. An attempt for thesimultaneous synthesis and sintering method aims for eliminating poresin the ceramic material to be synthesized through the SHS method by thepressure and manufacturing a dense sintered material in several secondsand it has been reported that a TiB₂ sintered material was manufacturedunder a pressure of 3 GPa only by electric ignition to a pressed mixtureof Ti (titanium) and B (boron). The relative density and the hardness ofthe sintered material were 95% and 2000 kg/mm² respectively.

Further, as another short time sintering method with application ofpressure, a method of densifying and a compacting ceramic material bythe combination of Explosive Shock Compaction method with the SHS methodhas been proposed (refer to U.S. Pat. No. 4,655,830 and Advanced CeramicMaterial, Vol. 3, No. 3, p 288-90 (1988)). This method conductssimultaneous synthesis and sintering (type I) or explosure shockcompaction and post-shock-heating (type II) of ceramic under a highimpact pressure of about several tens GPa in a short period of time. Forexample, in a micro sec order and it has been reported that a sinteredTiC material was synthesized starting from a powder mixture of Ti andcarbon as raw material under application of an impact pressure greaterthan 45 GPa in the type I. The resultant sintered material thus isrelatively porous and has micro-hardness of 500-700 kg/mm². It has beenalso reported that TiC-Al₂ O₃ composite ceramic was synthesized by theapplication of an impact pressure of 45 GPa from a powder mixture ofTiO₂, carbon and aluminum as the raw material. The micro-hardness of theresultant sintered material was 500 to 700 kg/mm², intergrain bondingwas relatively weak and fine cracks were observed in some places. As thetype II method, there has been reported, for example, that the compoundexothermic reaction is added for the post-shock-heating element to theshock compaction method for SiC ceramic, in which shock-compaction andpost-shock-heating were conducted in a structure comprising a sinteredSiC ceramic molding product sandwiched with molding pellets of a mixtureof Ti and C. The resultant sintered SiC material had a relative densityof 99% and a micro-hardness of 28 GPa.

Further, a 2-step sintering method improved from the conventionalsintering method has tended to attract an attention again in recentyears for the improvement of single phase ceramic, in particular, thehomogenity of the fine texture although this is a densification processrequiring a long time reported by (L. C. De. Jonghe, et al). The featureof this method resides in applying a heat treatment at a low temperature(a temperature at which sintering does not proceed substantially) forhomogenizing the fine texture preceding to the heat sintering step, andit has been reported that the homogenity of the fine texture wasremarkably improved by this low temperature homogenizing treatment.However, a considerable portion of the mechanisms is still not apparentat present and a working example thereof is restricted to oxide ceramicslike that in the plasma sintering method described above.

On the other hand, from a view point of the development for the powderprocessing technique, in the recent study related to the densificationof the ceramic and the control of the fine texture, studies have beenmade vigorously on synthesis and sintering of super fine ceramic rawmaterial powder by a gas phase technique such as plasma synthesis andsynthesis and sintering of super fine ceramic raw material powder by aliquid phase method such as a sol-gel method. There has been reportedthat refinement of the covalent bonding super fine SiC ceramic particlesmanufactured by a R. F. Plasma CVD process in the sintering step couldbe promoted considerably with no addition of raw aids (B, C, Al, Be), bythe refinement of the raw material powder and the development for theproduction of composite raw material powders and binder-containing rawmaterial powder (Pre-Print for the Lecture of the Ceramic Society, Part,No. 1, p 427-428, 1986). It is considered that the phenomenon is causedas a result of the promoting effect for the diffusing reaction developedin the sintering step due to the increase of the powder ceramic activityand the increase of the specific surface area or the like by therefinement of the raw material powder. In the sintering of the SiCceramic powder produced by plasma synthesis, it has been reported thatthe growth of the crystal grain size could be retained to about severalmicronmeters even if the hot press sintering temperature was elevated upto 2200° C.

As another example of applying an active plasma-synthesized powder and asuper fine powder produced by the gas/solid phase heterogeneous reactionregarding the promotion of densification, controllability for the finetexture and the improvement of the homogenity in less burnable ceramics,a result of the densification due to vacuum sintering and hot presssintering of TiB₂ ceramic has been reported (Journal of the AmericanCeramic Society, Vol. 67, No. 3, p 207-212 and Advanced CeramicMaterials, described above, Vol. 1, No. 1, 1986, p 50-54). In both ofthem, the raw material powder is an agglomerate powder with a grain sizeof less than 1 μm and an extremely active powder. In the vacuumsintering for the plasma-synthesized powder, increase of the relativedensity to 98-99% was attained by sintering at 1800°-2300° C. also withthis less sinterable TiB₂ ceramic. On the other hand, in the hot presssintering of a super fine TiB₂ powder produced by a solid/gasheterogenous reaction, a sintered material with sufficiently controlledfine texture having a relative density of greater than 99% and a crystalgrain size of 2 um was obtained under the presence of a slight amount ofFe and Ni (up to 0.4% in total).

Referring collectively to various kind of recent techniques tried forovercoming the foregoing problems, the technique for the sinteringprocess is, for example, as described below.

(A) The current supply sintering method is effective for conductiveceramic or cermet but it is not applicable to insulative orsemiconductive ceramic. Further, it is extremely difficult tosimultaneously attain sufficient control for the crystal grain size andthe ceramic constituent phase together with the densification.

(B) In the various kind of plasma sintering methods, for example,sintering of Al₂ O₃ ceramic using R.F. plasmas, there is a difficultythat the heat sintering temperature for the material to be sintered byor plasma heating greatly depends on the deposited amount of water,characteristics of the sintered material vary greatly and the like.Further, in this sintering method, the stability of an oscillatoroutput, sintering time, as well as the stability of gas flow rate andgas pressure of Ar, N₂ and H₂ or the like give a remarkable effect onthe density and the fine texture of the sintered material and it isdifficult to simultaneously attain both the high density and thehomogenous fine texture and the modification of the fine texture over awide range.

(C) In the microwave sintering method, there is a principle barregarding the selectivity for the sintering material that thesinterability is determined depending on the microwave interaction, thatis, the degree of absorption by the powder to be sintered. That is,there is a drawback that a conductive ceramic material reflectsmicrowaves making it impossible for heat sintering. Accordingly, for thenature of the material to be sintered, there is a difficulty in view ofthe design for the sintering material that selectivity is given only toa low loss insulator transparent to the microwave or a combination of alow loss insulator with a lossy insulator as an absorber.

(D) In the high-pressure self-combustion sintering method, sincesynthesis and press-sintering proceeds without external heating but onlyby the own compound exothermic reaction, extremely high temperature isformed during sintering synthesis to promote degasing from each ofelements, by which the sintered material generally tends to becomeporous along with the reduction of the pressure. For reducing of theporosity, manufacture of cermet (ceramic phase+metal phase) sinteredmaterial has been tried in recent years (Summary of the Proceeding inAutumn Meeting, Society of Powder and Powder Metallurgy, 1986, p 42-43).

In addition, since the synthesis temperature is extremely high and thereaction rate is generally high owing to a so-called self combustionmode of conducting sintering synthesis mainly with the inter-elementmixing of compounds forming extremely great heat of reaction, the finetexture is unimeaningly determined depending on the temperature of thesynthesis reaction and the cooling rate determined by the heat formed bythe inter-element reaction, making it difficult to control the finetexture over a wide range. Further, in the production of a compositeceramic phase, it is difficult to control the constituent phasedepending on the purpose, even if the thermodynamic stability of theceramic phase is taken into consideration.

(E) In the method of manufacturing a composite ceramic using the SHSmethod under an impact shock pressure, combustive sintering synthesis ispossible owing to a sufficiently high pressure caused by impact shockwaves and high temperature between particles even if the SHS reaction isnot self-sustaining. However, since the time of applying the pressure isas short as micro seconds and generation of high temperature is mainlylocalized on the surface of the particles, it still involves a drawbackthat the SHS reaction is not completed or the sintered material isliable to be destroyed due to the occurrence of cracks during rapid (μsec. order) pressure elimination. Further, due to the property of theself-combustion mode like that in the high-pressure self-combustionsintering method, and the short time pressure application in the orderof micro seconds it is difficult to control the fine texture over a widerange even by the use of post-shock heating.

(F) For the 2-step sintering method, a considerable portion of themechanism is not still apparent and, in many of reported examples, theanisotropy of the crystal structure of the material and the anisotropyfor the grain growth during sintering are moderate. However, since thisis a sintering method requiring a long time, even when a homogenzingheat treatment is applied at a low temperature thereby conductingdensifying heat sintering, it has a difficulty that the homogenity ofthe fine texture is tended to be lost depending on the heating rate andthe sintering time from the temperature for the homogenizing heattreatment to the main sintering temperature. In particular, in thesintering of less sinterable ceramic having a strong anisotropy, sincethe range for selecting the optimum heating parameters in the mainsintering step is extremely narrow, it is almost impossible at presentto attain the homogenity of the fine texture, versatile control anddensification.

(G) In the homogenizing sintering method aiming for the densificationand the control for the fine texture of the ceramic using aplasma-synthesized powder, powder synthesized by solid/gas phaseheterogenous reaction and ceramic powder by the sol-gel method resultingfrom the development of the powder processing technique, since thepowder becomes active due to super fine powderization, a great amount ofinevitable impurities are introduced in non-oxide series ceramics andthe moldability is reduced remarkably both for the oxide series andnon-oxide series ceramics, use of the molding aid or the like isessential, tending to cause scattering in the micro molding density.These drawbacks give a significant effect on the promotion of thedensification and the control for the fine texture in the sintering stepto leave various problems such as bubbles are left or causing abnormalgrain growth is brought about due to the inevitable impurities in thelow temperature sintering that utilizes the powder activity.

SUMMARY OF THE INVENTION

An overall object of the present invention is to provide a method ofmanufacturing a novel sintered ceramic material capable of sufficientlyattaining three subjects, that is, densification, control of a finetexture over a wide range and securing of homogenity that could not bedissolved in the prior art.

Another object of the present invention is to provide a method ofmanufacturing a sintered ceramic material capable of remarkably reducingheating energy by utilizing heat of chemical reaction as a heat sourcefor sintering.

A further object of the present invention is to provide a method capableof conducting rapid heating and manufacturing a sintered ceramicmaterial of dense fine texture by suppressing grain growth.

A further object of the present invention is to provide a method capableof manufacturing a sintered ceramic material which is homogenous andexcellent, for example, in abrasion resistance, heat resistance,corrosion resistance and oxidation resistance.

A further object of the present invention is to provide a method ofmanufacturing a sintered ceramic material capable of modifying a finetexture variously under the condition of a constant density and alsoimproving the various properties of the resultant sintered material evenin the sintering of a less sinterable ceramic, without adding asintering aid or fine texture controlling agent.

A further main object of the present invention is to provide a method ofmanufacturing a sintered ceramic material capable of improving thereliability and the performance of the sintered material through themodification of the underlying texture at an extremely micro level,without changing the fine texture of the sintered material and thehomogenity of the fine texture, subsequent to the ceramic sinteringstep.

A further overall object of the present invention is to provide a methodof manufacturing a novel sintered composite ceramic material byutilizing heat generated in a thermit reaction that can sufficientlyattain the three subjects, that is, densification, refinement for thetexture and the control for constituent phases, which could not beovercome in the prior art.

A further object of the present invention is to provide a method ofmanufacturing an excellent sintered composite ceramic material in whichthe ceramic constituent phases in the sintered material are sufficientlycontrolled by extending the range for the kind of thermit reactionsutilized, thereby sufficiently controlling the amount of heat applied bythe heat generation and applying heating in a short period of time.

A yet further main object of the present invention is to provide amethod of manufacturing a composite sintered ceramic material capable ofimproving the performance and the reliability through the reduction ofextremely fine pores without changing the fine texture, phaseconstitution, structure or the like of the sintered material, subsequentto a manufacturing step of the sintered composite ceramic material byutilizing the heat generated in the thermit reaction.

A still further overall object of the present invention is to provide asintered ceramic material and a sintered composite ceramic materialhaving a dense fine texture, excellent in various mechanical andchemical properties and extremely useful in the conventional field ofceramic utilization, as well as in various industrial fields requiringhigher performance.

For attaining the foregoing objects, in the first aspect of the presentinvention, there is provided a method of manufacturing a sinteredceramic material, comprising a step of pre-heating a powder to besintered at a predetermined temperature for a predetermined period oftime previously and a step of heat-sintering the powder to be sinteredwhich has been subjected to the pre-heating by heat generated in athermit reaction under pressure.

According to another mode of the above-mentioned method, there isprovided a method of manufacturing a sintered ceramic material, whereinthe sintering step by the heat generated in the thermit reaction isfollowed by heating at a predetermined temperature for a predeterminedperiod of time in order to improve the reliability and the performanceof the sintered material without changing the fine texture and thehomogenity of the fine texture of the resultant sintered ceramicmaterial.

According to the second aspect of the present invention, there isprovided a method of manufacturing a sintered composite ceramicmaterial, in which a mixture comprising

(A) at least one ceramic powder,

(B) at least one non-metallic powder selected from the group consistingof carbon, boron and silicon powders, and

(C) a metal powder and/or a non-metallic powder other than thatmentioned powder (B) above is sintered under heating by heat generatedin a thermit reaction under pressure.

In a preferred mode of a method of manufacturing a sintered compositeceramic material, a thermit composition for heating the powder mixtureto be sintered comprises a mixture of a copper oxide powder and analuminum powder or a combination of the mixture with a mixture of aniron oxide powder and an Si powder.

In another mode of a method of manufacturing a sintered compositeceramic material, there is provided a method of manufacturing a sinteredcomposite ceramic material, in which the sintering step for thecomposite ceramic utilizing the heat generated in the thermit reactionaccording to the method described above is followed by post-heating,preferably, a hot isostatic press or hot press treatment at atemperature of 500° to 1700° C. under a pressure within a range from 200to 2000 atm for 5 to 60 min., for the reduction of the porosity and theimprovement of the reliability and the performance of the resultantsintered composite ceramic, without changing the fine texture, phasecomposition, structure and the like thereof.

Other objects, as well as advantages and features of the presentinvention will become apparent to those skilled in the art by thefollowing detailed descriptions.

BRIEF EXPLANATION OF THE DRAWINGS

In the appended drawings,

FIG. 1 is a schematic vertical cross sectional view for a main portionof a high pressure generating device used for manufacturing a sinteredmaterial in Examples 1 to 3 to be described later.

FIG. 2 is an electron microscopic photograph showing a fine texture of asintered TiB₂ ceramic material with no addition of a binder obtained inExample 1, in which the photograph (A) shows a fine texture of asintered material obtained only by the heating in the thermit reactionunder 20,000 atm from a TiB₂ powder without applying preliminary vacuumheat treatment as a comparative example, and photographs (B), (C) and(D) show, respectively, a fine texture for each of sintered materialsmanufactured by the heating in the thermit reaction under 20,000 atmafter applying preliminary vacuum heat treatment each at a temperatureof 1300° C., 1500° C. and 1700° C. under a vacuum degree of 2×10⁻¹⁰ to1×10⁻⁴ torr.

FIG. 3 is a schematic vertical cross sectional view for a main portionof a super high pressure device used for manufacturing a sinteredcomposite ceramic material in Example 5 to be described later.

FIG. 4 is a schematic vertical cross sectional view for a main portionof a pressure device of a piston-cylinder type used for manufacturingthe sintered composite ceramic material in Example 6.

FIG. 5 is an electron microscopic photograph showing the fine texture ofthe sintered composite ceramic materials obtained by Example 5 and aconventional method, in which photograph (A) shows a fine texture ofsintered TiB₂ -Ni-B material obtained by a high pressure sinteringmethod in the prior art and photograph (B) shows a fine texture ofsintered TiB₂ -NiB composite ceramic material obtained by the method formanufacturing the sintered composite ceramic material according to thepresent invention.

FIG. 6 is an electron microscopic photograph showing a fine texture of asintered composite ceramic material obtained by Example 6 and aconventional method, in which photograph (A) shows a fine texture ofsintered Cr-Ni-B series composite material obtained by the vacuumsintering method in the prior art, photograph (B) show a fine texture ofa sintered composite material of an identical composition heat-sinteredby a thermit reaction under 2000 atm in not self-sustaining SHS reactionand photograph (C) shows a fine texture of a sintered composite ceramicmaterial obtained by a method of manufacturing (under 2000 atm) asintered composite ceramic material according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the first aspect of the present invention, there isprovided a method of manufacturing a sintered ceramic material capableof simultaneously attaining densification, control for the fine textureover a wide range and the homogenity of the fine texture that isextremely difficult to attain by the usual sintering method.

As described above, densification, control for the fine texture andimprovement of the homogenity in the sintered ceramic material have beenconsidered to be an important problem to be overcome in a closerelationship with the characteristics of the sintered material assubjects that conflict with each other in the sintering method of theprior art. The sintering method according to the present invention hasbeen deviced so as to overcome the subjects conflicting to each other,i.e., high densification, control of the fine texture over a wide rangeand improvement of the homogenity, even in a state where a densificationpromoter, fine texture controlling agent and the like are not present,which were almost impossible in the conventional sintering and itestablishes a new ceramic sintering method by overcoming them with asimple method comprising a step of applying a pre-heating treatment to apowder to be sintered and a step of applying heat-sintering for a shortperiod of time by utilizing the heat generated in a thermit reactionunder pressure to the powder to be sintered which has been subjected tothe pre-heating treatment described above.

The pre-heating is conducted, preferably, by vacuum heating or by anatmospheric heating in an inert atmosphere such as N₂, Ar, Ar+H₂ oratmospheric air, a reducing atmosphere or an oxidative atmosphere, at atemperature generally higher than 500° C., although it varies dependingon the kind of ceramic powder used or the like and the pressurecondition upon heat-sintering is properly selected depending on the kindof the ceramic powder used or the like and a pressure within a rangehigher than 200 atm is usually selected.

In the method of manufacturing the sintered ceramic material accordingto the present invention, a powder to be sintered which has beensubjected to a pre-heating treatment by vacuum heating or atmosphericheating is heated under pressure by a chemical reaction capable ofconveniently utilizing heating at high temperature in a short period oftime, with a so-called thermit composition. An example of the chemicalexothermic reaction for heating the powder to be sintered at a hightemperature for a short period of time by using the thermit compositionis shown, for instance, by the following equation.

    Fe.sub.2 O.sub.3 +2Al→Al.sub.2 O.sub.3 +2Fe+204 kcal(1)

A method of sintering a ceramic powder, a metal powder or a mixture ofthe ceramic powder and the metal powder by using heat generated in thethermit reaction as a heating source is disclosed in Japanese PatentLaid-Open Sho 61-186404 and U.S. Ser. No. 928,220. However, there hasnot yet been developed a ceramic sintering method capable of improvingthe fine texture that gives a remarkable effect on the characteristicsof the sintered material over a wide range and improving the homogenityand the reliability simultaneously with densification by previouslyapplying a pre-heating treatment to a powder to be sintered at apredetermined temperature for a predetermined period of time by vacuumheating or atmospheric heating and then applying rapid sintering underpressure for a short time by the heat of the thermit reaction.

The merit of the pre-heating in vacuum or atmosphere in the method ofmanufacturing the sintered ceramic material according to the presentinvention is that the activity at the surface of the ceramic powder,inevitable impurities and the like can be controlled variously, as wellas that sintering reaction can be partially started, depending on thecase, thereby enabling to various control the free energy of the ceramicpowder, which is the driving power for the sintering.

Accordingly, the method of the present invention has an excellent meritcapable of optionally modifying the characteristics of the powder to besintered, which are determined unimeaningly, for example, by the variouscharacteristics of the raw material powder, the blending composition,mixing condition, molding property, etc. of the ceramic powder, inparticular, by the powder processing technique in the conventionalmethod. It is possible to manufacture a homogenous and highly reliablesintered ceramic material with constant degree of density and a finetexture modified over a wide range, which are excellent features notfound in the prior art, by combining the powder to be sintered obtainedby the vacuum or atmospheric pre-heating for the powder to be sinteredwith rapid heating by the thermit reaction under pressure as anotherfeature of the ceramic sintering method according to the presentinvention. In addition, it is of course possible, as has been describedabove, to attain densification, control for the fine texture over a widerange and improvement of the homogenity, which were almost impossible inthe conventional sintering method, also for a monolithic ceramic thattends to cause abnormal grain growth under a state where a densifyingpromoter, a fine texture controlling agent or the like is not present.

As has been described above, since the pre-heating treatment to thepowder to be sintered and the short time heat-sintering by utilizing theheat generation in the thermit reaction are combined in the method ofmanufacturing the sintered ceramic material according to the presentinvention, the following remarkable effects can also be obtained inaddition to the main effect described above.

It is possible to modify the fine texture variously and thecharacteristics of the resultant sintered material can also be improvedunder the condition of a constant density even in the sintering for aless sinterable ceramic with no addition of a sintering aid or a finetexture controlling agent.

Since the heat of chemical reaction is utilized as a heat source forsintering, heating energy can be reduced remarkably.

It is possible to manufacture a new ceramic material by rapidheat-sintering by the selection for the kind and the blend of theceramic powder.

In another mode of the method of manufacturing the sintered ceramicmaterial according to the present invention, post-heating treatment isapplied subsequent to the ceramic sintering step, without changing thefine texture of the sintered material and the homogenity of the finetexture. As shown in Example 4 described later, various physicalproperties of the sintered material, such as hardness, heat diffusioncoefficient and fracture toughness are improved by the post-heating.However, no meaningful difference is recognized for the shape and thesize of the grains in the fine texture of the sintered material beforeand after the post-heating and, accordingly, it is supposed that theeffect of the post-heating is applied through the modification of theunderlying texture at an extremely micro level, which can improve thereliability and the performance of the sintered material.

The post-heating is also conducted, preferably, by vacuum heating oratmospheric heating such as an inert atmosphere, a reducing atmosphereor an oxidative atmosphere like that in the pre-heating described above.Further, it is applied to the sintered material under a pressurizedstate of 1 to 2000 atm, preferably, by a hot isostatic press, hot pressor heat treatment under a normal pressure atmosphere. The temperature inthe post-heating is within such a range as not changing the fine textureand the homogenity of the fine texture in the resultant sintered ceramicmaterial, which is properly selected depending on the kind or the likeof the ceramic powder used, within, a range not higher than the meltingpoint thereof.

In this way, it is possible to manufacture a ceramic material possessinga homogenous fine texture and of high performance and reliability byapplying the post-heating subsequent to the ceramic sintering step.

With the method of manufacturing the sintered ceramic material accordingto the present invention, it is possible to manufacture a ceramicmaterial which is extremely useful industrially. For instance, thesintered TiB₂ materials with no addition of binder shown in examplesdescribed later can be used as high temperature structural components,corrosion resistant components and electric supply parts due to theirexcellent abrasion resistance, heat resistance, corrosion resistance,oxidation resistance and conductivity. Meanwhile, sintered β-Si₃ N₄material with no addition of binder can be applied as cutting tools,molten aluminum resistant components and various kind of components forchemical plants in view of their high hardness, excellent oxidationresistance, chemical reaction resistance and heat resistance.

It is of course possible to previously apply a preliminary moldingtreatment for the powder to be sintered by various kind of lump moldingmethod as such as die molding, cold isostatic molding method and hotisostatic molding method, prior to each of the treatments.

The powder to be sintered applicable to the sintering method by thefirst aspect of the present invention is not restricted to ceramicpowders in a restricted meaning and it should be construded in a morebroad meaning. That is, the sintering method described above isapplicable not only to various kind of so-called ceramic powders in therestricted meaning but also to a mixture comprising a combination ofsuch a powder properly with various kind of non-metallic powders such asof carbon, boron and silicon and various kind of metal powders such asnickel, chromium, vanadium, niobium, tantalum, molybdenum, tungsten andmanganese.

On the contrary, in the method of manufacturing the sintered compositeceramic material according to the second aspect of the presentinvention, a specific mixture to be sintered is used, in which thethermit heat is utilized and conditioning and selection of the ceramicpowder to be sintered are skillfully deviced in order to obtain asufficiently controlled fine texture for the constitution of the finalcomposite phase, which could not be manufactured so far in the prior artmethod.

That is, in the method of manufacturing a sintered composite sinteredmaterial according to the present invention, a mixture comprising (A) aceramic powder, (B) a non-metallic element powder selected from thegroup consisting of carbon, boron and silicon and (C) a metal powderand/or a non-metallic element powder different from that in (B) above isheated with a chemical reaction capable of indirectly attaining heatingat high temperature in a short period of time, that is, so-calledthermit composition. A method of sintering a ceramic powder, metalpowder or mixture of a ceramic powder and a metal powder using heatgenerated in the thermit reaction as a heat source is disclosed inJapanese Patent Laid-open Sho 61-186404 and U.S. Ser. No. 928,220 asdescribed above. However, it has not yet been developed a highlycontrolled technique for manufacturing a composite ceramic in which thenon-metallic element powder and the metal powder and/or non-metallicelement powder different from the former present in the powder mixtureto be sintered forms a new ceramic phase due to the heat of the thermitreaction and the resultant composite ceramic material can be densifiedunder sufficient control of the phase constitution and the fine texturein accordance with the purpose by proper mixing of the ceramic powder,the non-metallic element powder, metal powder or the non-metallicelement powder different from the former.

The merit of the thermit heating in the manufacture of the compositeceramic as in the method of the present invention resides in thatformation of a new ceramic phase is promoted by the instantaneousbonding of the non-metallic element powder and the metal powder and/orthe non-metallic element powder different from the former, by applying agreat amount of heat to the powder mixture to be sintered in a shortperiod of time. For instance, although the resultant phase is changeddepending on the diffusing phenomenon in the synthesis of the ceramicphase by the conventional method, a sintered composite ceramic materialcan be prepared while rather optionally controlling the finalconstituent phase by properly selecting the initial composition of thepowder mixture. In the method of manufacturing the sintered compositeceramic material using the thermit heating, it is possible tomanufacture a sintered composite ceramic material which is dense and notfound in the prior art, while forming an aimed new ceramic phase bycontrolling the amount of the intermediate product and stabilizing thephase composition with excellent feature of the raw ceramic powder, forexample, grain size being maintained as it is.

In Japanese Patent Laid-Open Sho 61-186404 described above, it isdisclosed that a thermit composition using iron oxide as an oxide andAl, Si, Ti, Mg, Ca and the like as the reducing metal powder iseffective. An example of the reaction is shown by the formula (1)described previously.

On the contrary, a thermit composition suitable to be used in the methodof manufacturing the sintered composite ceramic material according tothe present invention utilizes the reaction:

    3CuO+2Al→Al.sub.2 O.sub.3 +3Cu+289 kcal             (2)

as a heat source of the main chemical reaction and has a merit that thereaction heat which can be taken out effectively per one mol of Al isincreased by about 40% as compared with the Al thermit composition inthe chemical reaction show in the above-mentioned formula (1). Uponigniting the thermit composition, the ignition energy can be saved byusing a Si thermit composition (a mixture of one mol of an Si powder and2/3 of an iron oxide) together, and the ignition electric power bycurrent supply heating, for example, by a heater can also be reduced. Inthe method of manufacturing the sintered composite ceramic material, thethermit composition is not limited only to the mixture of the copperoxide powder and the aluminum powder described above but various kind ofthermit composition mixtures disclosed in Japanese Patent Laid-Open Sho61-186404 can of course be utilized as the sintering heat source. Athermit composition comprising a mixture of the copper oxide powder andthe aluminum powder is most suitable in that the amount of heat from thechemical reaction that can be effectively taken out per charged amountof the thermit composition.

Also in the method of manufacturing the sintered composite ceramicmaterial, it is possible, as another mode, to apply the post-heatingtreatment as described above for reducing the porosity and improving thereliability and the performance without changing the fine texture, thephase composition, the structure and the like of the resultant sinteredcomposite material, subsequent to the sintering step for the sinteredcomposite ceramic material by utilizing the heat generation in thethermit reaction. The post-heating treatment in this case is conducted,preferably, as a hot isostatic press or hot press treatment for 5 to 60min at a temperature of 500° to 1700° C. and under a pressure within arange of 200 to 2000 atm.

In the method of manufacturing the sintered composite ceramic material,the pre-heating treatment to the powder to be sintered may not beapplied, in which similar effect as described above can also beobtained. However, it is of course possible to apply the pre-heatingtreatment and obtain the effect thereby.

As has been described above, in the method of manufacturing the sinteredcomposite ceramic material according to the present invention, it ispossible to manufacture a high quality and novel ceramic material inwhich the fine texture, the phase constitution or the structure and thephase distribution of the sintered composite ceramic material aresufficiently controlled, which are extremely difficult to manufacture inthe usual sintering method. That is, in the method of manufacturing thesintered composite ceramic material according to the present invention,since rapid heating under pressure is possible and a portion of thepowder to be sintered forms a ceramic phase under the control oftemperature, pressure and time, the composition of the ceramic phase,phase structure, distribution for each of the phases and the like can becontrolled sufficiently. In addition, there are also obtainable aneffect that the heating is conducted for a short period of time and thegrain growth or the like can be suppressed and a merit that the heatingelectric power charged for sintering can be remarkably reduced.

Further, for attaining such effects, combined use of the sinteringmethod for extremely short time by the effective utilization of the heatof the thermit reaction and the skillful controlling method for thepowder to be sintered gives an extremely effective means, therebyenabling to manufacture various kind of new sintered composite ceramicmaterials. As a result, it is possible to manufacture a material whichis extremely useful industrially. For instance, the sintered TiB₂ -NiBmaterial shown in the example was not obtained as a pure 2-phasesintered composite material under the control of the fine texture in theconventional method, and it is applicable to high temperature glassmolding parts by utilizing its excellent heat resistance and glassresistance. On the other hand, the sintered B₄ C-TiB₂ material can beapplied, for example, to cutting tools and molten aluminum meltingcomponents due to its excellent high hardness and oxidation resistanceor aluminum resistance. In addition, the sintered Si₃ N₄ -SiC compositematerial can also be applied as high temperature structural componentsin view of its homogenity for the fine texture.

DETAILED DESCRIPTION FOR PREFERRED EMBODIMENTS

The method according to the present invention will now be describe morespecifically referring to examples but it will be apparent that thepresent invention is limited to the following examples.

EXAMPLE 1

1.3 g of a TiB₂ powder 1 μm in average grain size (manufactured byHermann C. Starck Co.) was cold molded into a 12.8 mm disc-shape byusing a CIP (cold isostatic pressing) device. The powder molding productwas applied with a preliminary vacuum heat treatment by using a vacuumheating furnace. The temperature was set to 1300° C., 1500° C. and 1700°C., respectively, and the products were maintained under a vacuum degreeof 2×10⁻³ to 1×10⁻⁴ torr each for three hours and then cooled to formpowder lumps to be sintered.

As a thermit composition, 34.2 g of a thermit composition prepare bymixing an iron oxide powder and an Al powder at 1:2 molar ratio wasdivided into two portions and then cold molded each to a size of 30 mmdiameter, while 7.7 g of the identical composition was cold molded intoa cylindrical form with 30 mm outer diameter and 22 mm inner diameter.Then, the TiB₂ lump to be sintered which had been subjected to thepreliminary vacuum heating was disposed by way of a thin layer ofhexagonal system boron nitride between the two discs and the cylindermade of the thermit. The assembly was placed in a belt-like highpressure generating device.

FIG. 1 shows a disposition state in the high pressure generating device.Reference numerals 1 and 2 represent, respectively, a cylinder and ananvil to constitute a high pressure generating vessel. 3 denotes agasket made of pyrophilite for sealing under pressure. 4 denotes a heatinsulator made of pyrophilite. 5, 6 and 7 denote, respectively, a copperplate, a steel ring and a molybdenum plate 8 denotes a ceramic heatinsulator, and they constitute an assembly for supplying electriccurrent to a cylindrical carbon heater 9. 10 denotes a heat insulatormade of pyrophilite, 11 denotes a thin layer of hexagonal system boronnitride which is served for preventing reaction between the thermitcomposition 12 comprising the iron oxide and the aluminum powder, and apowder lump 13 to be sintered applied with preliminary vacuum heattreatment and for giving electric insulation to the carbon heater 9. Thehigh pressure generating device described above is operated as below. Aload is applied to opposed anvils 2 to generate a pressure of 20,000 atmto a specimen chamber. Electric current is supplied from the opposedanvils 2 to the carbon heater 9 to heat the specimen chamber to830°-920° C. and the thermit composition 12 was ignited. The amount ofheat generated in the thermit in this example was 40 kcal.

Heat sintering for a short period of time to the TiB₂ powder to besintered heated in vacuum by the heat of the thermit reaction wascompleted at the instance the change of the distance between the anvilswas terminated and, after cooling of the specimen portion, the pressurewas removed and the sintered TiB₂ ceramic material was recovered. Thepowder to be sintered applied with preliminary vacuum heating shown inthis example formed a dense sintered material and had a relative densityof greater than 99%.

FIG. 2 shows the result of the observation for the fine texture by anelectron microscope on the sintered TiB₂ ceramic material obtained inthis example. The photograph (A) shows the fine texture of the sinteredmaterial manufactured only by the sintering of the thermit heating forthe TiB₂ powder not applied with the preliminary vacuum heat treatment,in which grain growth does not occur at all and a fine sintered materialkept at an average grain size of 1 μm was obtained. Photographs (B)-(D)show the fine texture of the sintered material after applying sinteringby thermit heating for the TiB₂ powder to be sintered which had beenapplied with preliminary vacuum heat treatment according to the presentinvention. The photograph (B) shows the fine texture of the powder to besintered applied with the preliminary vacuum heat treatment at 1300° C.for 3 hours, in which TiB₂ grains have a structure comprising uniformrod-like particles with 2 to 3 μm length entangled to each other, and aform that clearly exhibits the anisotropy of the TiB₂ ceramic crystalsis obtained. The photograph (C) shows the fine texture sintered bythermit heating in a case of applying the preliminary vacuum heattreatment at 1500° C. for an identical, period of time, which has afeature in a structure comprising isotropic crystal forms and withuniform crystal grain size of 1 to 2 μm and a definite habit. Thephotograph (D) shows a case in which the preliminary vacuum heattreatment was applied at 1700° C. for an identical period of time, inwhich a fine texture comprising isotropic grains with uniform grain sizeof 2 to 5 μm having a definite habit could also be obtained.

As has been described above, the four kinds of sintered materials havesubstantially the constant density, clearly showing that the sinteringmethod can improve the fine texture over a wide range even for the rawmaterial of an identical composition at the same density while keepingthe homogenity. Diboride ceramic is one of typical examples of lesssinterable ceramics. For instance, in TiB₂ ceramic, densification andcontrol for the grain growth can be attained only in restrictedmanufacturing conditions even if the ultrafine raw material powder issufficiently conditioned and micro additive elements are utilizedeffectively as shown by the subjects left in the prior art, and it isimpossible to change the fine texture while keeping the density constantafter sintering. It was already reported, for instance, that if the finetexture of the sintered TiB₂ material is intended to be changed by theprior art, reduction of the density (remaining or pores) andinhomogenity of the fine texture (presence of various kind of crystalgrain sizes from several um to several tens μm) are inevitably caused,and the various characteristics are naturally changed, that is,deteriorated, and the reliability is also reduced remarkably(destruction thermal stress or the like).

As has been described above, various characteristics of the sinteredceramic material strongly depend on the change of the fine texture, andthe characteristics of various kind of sintered materials obtained bythis sintering method will be examined in comparison. For instance, whenthe heat diffusion coefficient and the heat conductivity at a roomtemperature are compared as the thermal nature of the TiB₂ ceramics, theheat diffusion coefficient changes as 0.22 cm² sec, 0.26 cm² /sec and0.34 cm² /sec respectively and the heat conductivity changes as 65W/m.K, 76 W/m.K and 91 W/m.K respectively in the sintered materialswithout heat treatment and applied with preliminary vacuum heattreatment at each of temperatures of 1300° C. and 1700° C. Further, TiB₂ceramic is known as a conductive ceramic and, when the specificresistivity at a room temperature is compared respectively like that inthe former, it changes as 11.3 μ.ohm-cm, 11.1 μohm-cm and 10.4 μ.ohm/cmrespectively. For the mechanical property of the sintered material,difference in various kinds of characteristics becomes apparent such asfracture toughness of the sintered material applied with preliminaryvacuum heat treatment at 1300° C. is considerably increased, and thereproducibility for each of the physical property values on everysintering was also excellent.

EXAMPLE 2

As a ceramic powder, 1.3 g of β-Si₃ N₄ powder with an average grain sizeof 0.65 μm and an α-phase ratio of 1% (manufactured by Hermann C. StarckCo.) as cold molded into a disc-like shape of 12.8 mm diameter. Thepowder molding product was subjected to a preliminary N₂ atmosphere heattreatment in a nitrogen atmosphere heating furnace while flowing a N₂gas at 100 ml/min. Heat treatment was conducted each for 4 hours whilesetting the temperature for the preliminary N₂ atmosphere heat treatmentto 1500°-1800° C. The powder material to be sintered was charged in thehigh pressure device shown in FIG. 1 and a sintered ceramic material wasmanufactured under the same conditions as those in Example 1 except forchanging the amount of heat generated from the thermit composition as 30kcal.

All of β-Si₃ N₄ sintered materials obtained in this example formedextremely dense sintered material and had a relative density of greaterthan 98to 99% even with no addition of a binder. When the fine textureof the resultant sintered material was observed under an electronmicroscope, grain growth was scarcely observed in the sintered β-Si₃ N₄material applied with the preliminary heat treatment at 1500° C. in theN₂ gas stream and the crystal grain size was not greater than 1 μm. Thefine texture of the sintered material from the β-Si₃ N₄ powder materialto be sintered applied with the preliminary heat treatment at 1800° C.in the N₂ gas stream comprised a structure in which uniform rod-likeparticles with 2 to 3 μm length were entangled to each other, which wassimilar to the fine structure obtained by the usual sintering methodwith addition of the binder. However, when compared with the sinteredmaterial by the usual method under the condition of the constantdensity, β-Si₃ N₄ particles had a remarkable feature in that they wereextremely homogenous and fine rod-like material and showed a significantdifference relative to the fine texture comprising β-Si₃ N₄ of variouskind of aspect ratioes grown coarsely as recognized in the usual method.

When characteristics of the sintered materials obtained in this examplewere compared, Vickers micro-hardness was 1800 kg/mm² in a case ofapplying the preliminary heat treatment at 1500° C. in N₂ gas stream,while it was 2000 kg/mm² in a case of applying the preliminary heattreatment at 1800° C. in the N₂ gas stream. When the heat conductivityof the sintered material was compared, the heat conductivity at a roomtemperature of the sintered material applied with the preliminary heattreatment at 1800° C. in the N₂ gas stream was more satisfactory thanthat of the sintered material applied with the preliminary heattreatment at 1500° C. in the N₂ gas stream.

EXAMPLE 3

As a ceramic powder, 3C structure SiC powder (manufactured by Hermann C.Starck Co.) was used and a small amount of boron B (less than 1%) wasused as a sintering aid. 1.3 g of a specimen was molded into a disc-likeshape with 12.8 mm of diameter by using a CIP device. The powder moldingproduct was applied with a preliminary heat treatment in a vacuumheating furnace. The vacuum heating temperature was set to 1400° C. to1600° C., while the heating time was set to 2 to 3 hours. The sinteredpowder material was charged in the high pressure device shown in FIG. 1,and a sintered ceramic material was prepared under the same conditionsas those in Example 1 except for changing the amount of heat generatedfrom the thermit composition to 35 Kcal.

All of the SiC sintered materials of 3C structure obtained in thisexample formed dense sintered material and had a relative theoreticaldensity of greater than 99%. As a result of examining the fine textureof the resultant sintered material, the fine texture of the sinteredmaterial of the B-added SiC powder to be sintered applied with thepreliminary vacuum heat treatment at 1400° C. for 2 hours had a featurein the homogenous and fine crystal grains with 1 to 2 μm size, while thefine texture of the sintered material of the same powder to be sinteredapplied with the preliminary vacuum heating at 1600° C. for 3 hours hada feature in the structure in which uniform rod-like particles withabout 3 μm length were entangled to each other. In the sintering for SiCceramic, a method of adding a slight amount of B, C or the like as thesintering aid is used since this is less sinterable ceramic, but is isalmost impossible to suppress the abnormal growth of SiC crystal grainsalong with densification for the sintered material which is nearly in anelemental form by the usual vacuum sintering or hot press sintering. Asa result, the fracture toughness of the resultant sintered material isunimeaningly determined depending on the fine texture.

As a result of this example, the fracture toughness of the sintered SiCmaterial comprising fine crystal grains, determined by micro indentationmethod, was 3 MN/m^(3/2) in a case of applying a preliminary vacuum heattreatment at 1400° C. for 2 hours but was increased up to 6 MN/m^(3/2)as a result of applying a preliminary vacuum heat treatment at 1600° C.for 3 hours.

EXAMPLE 4

For improving the reliability and the performance of the sintered TiB₂material obtained in Example 1, a HIP (hot isostatic press) treatmentwas applied in an Ar atmosphere under 200 atm. The HIP temperature wasset to 1450° C., while the pressurizing time was set to 30 min.

The micro Vickers hardness at a room temperature of thermit sinteredTiB₂ material applied with a preliminary heat treatment at 1700° C.shown in FIG. 2 (photograph (D)) was 22 GPa, whereas the hardness at aroom temperature after the HIP treatment was increased to 21-29 GPa. Inaddition, the heat diffusion coefficient at a room temperature was alsoincreased by about 10%. When the change of the fine texture was examinedin comparison before and after the HIP treatment by the electronmicroscope, no meaningful difference was recognized for the grain shape,size or the like. In addition, it could be confirmed that the homogenityof the fine texture was kept all over the sintered material. The sameeffect of the treatment was also confirmed in the hot press (HP)treatment in vacuum atmosphere or a heat treatment in Ar+H₂ atmosphere.Details for the processing conditions were at 1400° C. for one hour inthe hot press and at 1400° C. for 2 hours in the Ar+H₂ atmosphereheating. The processing pressure is different for the HIP treatment, theHP treatment and the atmospheric heat treatment. As a matter of fact,selection for the temperature and the processing time required for thechange of the underlying texture of the sintered material at anextremely micro level are different, without changing the fine texture.As the best mode, the HIP treatment or the HP treatment is preferredfrom an industrial economical point of view.

The fracture toughness of the sintered TiB₂ material applied with theHIP treatment had such a high value of 6 MPa.m^(1/2) that was notobtained in the prior art. In this post-heat treatment, test wasconducted for 20 specimens in total, in which the performance of thesintered material before and after the HIP treatment was improvedsubstantially at an identical level, showing the extremely highreliability of this post-heat treatment. Although details are not shownfor other ceramic materials, similar effects of this treatment wereconfirmed also for various other materials.

As shown in this example, the post-treatment such as HIP, HP oratmospheric heating for improving the reliability and the performance ofthe sintered material through the modification of the underlying texturewithout changing the fine texture of the sintered material and thehomogenity of the fine texture, subsequent to the sintering step for theceramic shown in Example 1 is an extremely effective means in themanufacture of high performance ceramics.

In each of the examples described above, explanation has been made, inparticular, to well-known diboride ceramics Si₃ N₄ and SiC as examplesof less sinterable ceramics in each of the examples, but the sinteringmethod according to the present invention is of course applicable toother various kind of ceramics, for example, covalent bonding ceramics(for example, AlN or SiBN), transition metal carbide series, silicideseries, oxide series, transition metal nitride series, phosphatizedseries, sulfide series or mixed series ceramics.

The subsequent examples concern a method of manufacturing a sinteredcomposite ceramic material according to the present invention byutilizing a thermit reaction.

EXAMPLE 5

After weighing 3.2 g of a TiB₂ powder of 1 μm in average grain size(manufactured by Hermann C. Starck Co.), 2.9 g of a Ni powder of 0.05 μmof average grain size (manufactured by Shinku Yakin Co.) and 0.64 g of aboron powder of 0.5 μm of average grain size (manufactured by Cerac Co.)and mixing them sufficiently, they were used as a raw mixture formanufacturing a composite ceramic.

1.85 g of specimen was sampled from the powder mixture and coldisostatically molded into a disc-like shape of 12.8 mm diameter toprepare a lump mixture to be sintered. As a thermit composition, 22.9 gof a thermit composition prepared by mixing a copper oxide powder and anAl powder at 3:2 molar ratio was divided into two portions and coldmolded into a size of 30 mm diameter, while 7.5 g of the samecomposition was cold molded into a cylindrical form with 30 mm outerdiameter and 22 mm inner diameter, and they were disposed by way of athin layer of hexagonal silicon nitride. 6 g of an Si thermitcomposition (a mixture of 1 mol of an Si powder and 2/3 mmol of an ironoxide powder which was the same also in subsequent examples) wasdisposed in adjacent with the disc-like molding product of the thermitcomposition with 30 mm diameter, and the assembly was charged in abelt-type high pressure generating device. FIG. 3 shows a dispositionstate in the high pressure generation device. Identical elements asthose for the high pressure generating device shown in FIG. 1 carry thesame reference numerals and descriptions for such elements are omitted.In FIG. 3, 11a and 11b show, respectively, a cylinder made of hexagonalsystem boron nitride crystals and a thin layer made of carbon black,which are served for preventing reaction and electric insulation betweena thermit composition 12 comprising the iron oxide and aluminum powdersand the Si thermit composition 14 and, a heater 9 and the powder lump tobe sintered (mixture of TiB₂, Ni and B powder) 13. A load was applied toopposed anvils to generate a pressure of 20,000 atm in the specimenchamber and, thereafter, electric current is supplied from the opposedanvils to the heater to heat the specimen chamber. In this example, ithas been found that the thermit compositions were ignited spontaneouslyin the order of the Si thermit composition and the Al thermitcomposition (powder mixture of copper oxide+Al) to release a greatamount of reaction heat by the local heating of the thermit compositiondue to the charge of the electric power of about 1 KW. The amount ofthermit heat generated in this example was 33.8 Kcal. The heat-sinteringfor the powder to be sintered by the thermit reaction heat was completedat an instance the change of the distance between the anvils wasterminated and, after the cooling of the specimen portion, pressure wasremoved and TiB₂ series sintered composite ceramic material wasrecovered by removing the pressure.

The composite powder to be sintered shown in this example formed a densesintered material and had a bulk density of 5.02 g/cm³. When thecomposite phase constituting the sintered material obtained by powderX-ray diffractiometry was identified, it could be confirmed that it wasa pure 2-phase sintered composite material comprising TiB₂ and Ni.

FIG. 5(B) shows the result for the observation of the fine texture inthe sintered composite ceramic material obtained in this example by wayof an electron microscope. It can be seen that no abnormal grain growthoccurs at all for TiB₂ ceramic particles and they are bonded extremelysatisfactorily by means of NiB ceramic while keeping 1 μm of the averagegrain size. In the TiB₂ -Ni-B series sintered composite ceramic materialin the prior art, no dense sintered material under such a sufficientcontrol for the fine texture and the phase constitution has not yet beenobtained.

FIG. 5(A) shows a fine texture of a sintered material with the identicalraw material obtained by a usual high pressure sintering method as ancomparative example. The sintering pressure is also 20,000 atm. In thehigh pressure sintering method, densification can be promoted bycompulsorily eliminating pores in the powder to be sintered by thepressure through a moderate heating step for the powder to be sintered.In the manufacture of the sintered material, however, the sinteringtemperature exceeds 1550° C. the grain growth of TiB₂ particles isremarkably accelerated along with the progress of the densification andthe grain size of the TiB₂ particles as the raw material is grown nearlyseveral to ten times during sintering. TiB₂ crystals are known asceramics having particularly strong crystallographic anisotropy, inwhich a TiB₂ /TiB₂ contact portions are increased by abnormal graingrowth to result in remarkable thermal stresses with respect to Niseries borides composited therewith, simultaneously, and also increasefracture sensitivity. On the other hand, it is also difficult to stablycontrol the phase of the Ni series borides unimeaningly in such asintering step with controlling factor in view of time.

TiB₂ -Ni-B series sintered material prepared by the high pressuresintering method shown in FIG. 5(A) was sintered at 1450° C. for 10 minunder a pressure of 20,000 atm for suppressing the abnormal grain growthof TiB₂ particles, but uniform dispersion of TiB₂ particles wasinsufficient and a great amount of pores substantially the same as TiB₂particles was observed. In addition, as a result of examining the phaseconstitution of the sintered composite material by powder X-raydiffractiometry, it consisted of three phases of TiB₂, NiB and Ni₄ B₃and it was extremely difficult to manufacture TiB₂ -NiB fine texture ofpure 2-phase as shown in FIG. 5(B) even how various sintering conditions(time, temperature and the like) were optimized.

The volumic ratio between TiB₂ -NiB sintered materials showing the finetexture in FIG. 5(B) was about 6:4. As other characteristics obtained,the micro Vickers hardness was 14.5 GPa, the average heat expansioncoefficient up to 800° C. was 7.7×10⁻⁶ /°C. and the heat conductivity at800° C. was 0.39 W/cm° C.

EXAMPLE 6

A CrB powder of 2 μm in average grain size (prepared by applyingadditional pulverization to the product manufactured by NipponShinkinzoku Co.), an Ni powder of 2 μm in average grain size and a boronpowder of 0.5 μm in average grain size were mixed each at 3.43 g, 2.81 gand 0.62 g in the same way as in Example 5, from which 2.1 g of specimenwas sampled and prepared into the same lump mixture to be sintered as inExample 5.

The lump mixture to be sintered was charged together with an Si thermitcomposition and an Al thermit composition comprising an Al powder and acopper oxide powder (total heat calorie of 33.8 Kcal) in apiston-cylinder type pressing device shown in FIG. 4 and pressurized to2,000 atm. Then, by supplying electric current to a electric supply wire25, the thermit compositions were ignited to prepare a sinteredcomposite material. In FIG. 4, are depicted a cylinder 21, a press punch22, a plate 23, a cylinder 24, an electric current supply wire 25, an Sithermit composition 26, and an Al thermit composition 27. 28 denotes anintervening layer for preventing the corrosion of the material to besintered by molten copper from the thermit composition and 29 denotes asample for the powder mixture to be sintered.

FIG. 6(C) shows the fine texture of the resultant sintered material.

According to the result of X-ray diffractiometry, the sintered materialcomprised CrB, (Cr, Ni)₃ B₄, (Ni, Cr)₄ B₃, (Ni, Cr)B phases, and thefracture toughness was about 6 MNm^(-3/2). It can be said to be aconsiderably excellent sintered material excepting that pores that seemto be bubbles of less than 2 μm are scattered in the sintered material.As a result of applying a heat treatment at 800° C. in an atmosphericair, the sintered material showed rather preferable oxidationresistance.

The fine texture of the sintered material shown in FIG. 6(A) and (B) arefine textures of Cr-Ni-B series sintered composite material in a case ofapplying the vacuum sintering method and the heat-sintering by theidentical thermit reaction in the not self-sustaining SHS reaction under2000 atm.

In the sintered material at an identical Cr/Ni/B ratio (raw material isa mixture of CrB of 2 μm and NiB of 1 μm) applied with vacuum sinteringat 1500° C., the (Cr, Ni)₃ B₄ phase caused abnormal growth byheat-sintering for one hour as shown in FIG. 6(A), by which the sinteredmaterial becomes extremely fragile (K_(IC) <2 MNm^(-2/3)). On the otherhand, a mixed lump to be sintered was prepared by mixing each ofelemental powders of Cr, Ni, and B by 5.1 g of Cr, 1.56 g of Ni and 1.35g of B in the same manner as in Example 5, from which 1.8 g of specimenwas sampled to prepare a mixed lump to be sintered and a sinteredcomposite material was manufactured under the same conditions as thosein Example 6. The feature of the sintering method resides in conductingsintering by using the thermit reaction heating to the SHS reactionwhich is not self-sustaining and proceeds entirely from element powder,but a great amount of bubbles of about 10 μm were formed as shown inFIG. 6(B) and it can not be said to be a dense sintered material. As aresult of comparing the fine textures of the sintered materials by eachof the methods, it is shown that the method of manufacturing thesintered material (FIG. 6(C)) in Example 6 is an extremely excellentmethod for manufacturing a sintered composite ceramic material.

EXAMPLE 7

A Ti powder of 3 μm in average grain size as a metal powder, a boronpowder of 0.5 μm as a non-metallic element powder and a TiB₂ powder of 1μm in average grain size as a ceramic powder were used and a mixedpowder to be sintered was prepared by using the mixing method as shownin Example 5 such that the volumic ratio of the mixture of the Ti powderand the boron powder (prepared such that the atom ratio between Ti andboron was 1/1) relative to the TiB₂ ceramic powder was 20%, from which 2g of specimen was sampled and cold isostatically molded into a disc-likeshape of 12.8 mm diameter to prepare a mixed lump to be sintered. As athermit composition, an Si thermit composition and a thermit compositioncomprising a mixture of an Al powder and an iron oxide were used (totalheat calorie of 43.8 Kcal), and charged in the piston-cylinder typepressing device shown in FIG. 4 to manufacture a sintered compositematerial in the same manner as in Example 6 under 2000 atm.

TiB₂ ceramic is known as a high melting less sinterable ceramic and itis extremely difficult to sinter under sufficient control for thedensity, fine texture and the phase constitution by the usual sinteringmethod. However, by using this method, the following sintered compositeceramic material, for instance, can be available which is dense andunder sufficient control for the fine texture.

When the phase constitution of the sintered material was examined bypowder X-ray diffractiometry, it was found to be a 2-phase sinteredcomposite material comprising the TiB₂ phase and the TiB phase. As aresult of examination by both of the means, that is, as a result of theX-ray diffractiometry and the result of the photoelectron spectroscopyby XPS, presence of unreacted Ti and B was not confirmed. Further, theTiB₂ particles were of about 1 μm size and grain growth was scarcelyrecognized.

EXAMPLE 8

In the same manner as in Example 7 except for using B₄ C of 0.8 μm inaverage grain size as a ceramic powder (manufactured by Denki KagakuCo.), Ti as a metal powder and boron as a non-metallic element powderwere mixed such that the volumic ratio of the mixture (prepared suchthat the atom ratio between Ti and B was 1/2) relative to the B₄ Cceramic powder, from which 1.5 g of specimen was sampled and preparedinto a disc-like mixed lump of 12.8 mm diameter to be sintered. Themixed lump to be sintered was charged in the high pressure generatingdevice shown in FIG. 3 and applied with a pressure of 20,000 atm andthen applied with a sintering treatment by the thermit exothermicreaction by the same combination of the Si thermit composition and themixture of the copper oxide powder and the Al powder.

The sintered material obtained in this example consisted of pure 2-phaseB₄ C and TiB₂ as a result of the powder X-ray diffractiometry and thepresence of other unreacted raw material or the like was not recognized.The resultant sintered composite material was dense in which the grainsize for each of B₄ C and TiB₂ was not greater than about 1 μm and thedispersion of B₄ C was uniform. The sintered material had a microVickers hardness of 3200 Kg/mm².

EXAMPLE 9

MoSi₂ of 2 um in average grain size (manufactured by Nippon ShinkinzokuCo.) was used as the ceramic powder, and carbon black of 0.01 μm inaverage grain size (manufactured by Cabott Co.) and amorphous silicon of0.1 μm (manufactured by Komatsu Densi Kinzoku Co.) were used as thenon-metallic element powder, and the volumic ratio of the mixture ofsilicon and carbon black to the MoSi₂ powder as set to 30%, 2.5 g ofspecimen was sampled from the mixture and molded into a disc-like shapeof 12.8 mm diameter to prepare a mixed lump to be sintered. The mixedlump to be sintered was charged in the high pressure device shown inFIG. 3 and a sintered composite ceramic material was manufactured underthe same conditions of the exothermic thermit reaction as in Example 8under a pressure of 10,000 atm.

The resultant sintered material was dense and grain growth of MoSi₂particles was scarcely observed. As result of the powder X-raydiffractiometry, a slight Mo₂ C phase was confirmed in addition to theMoSi₂, SiC (β-phase). In the manufacture of the sintered material, thecarbon black was mixed in excess by about 10% than 1/1 SiC/C ratio andthe presence of free Si was not observed. It was confirmed by a heatingexperiment in an atmospheric air at 1000° C. that the sintered materialwas satisfactory sintered material of excellent heat resistance andoxidation resistance.

EXAMPLE 10

Sintered composition ceramic material was manufactured under the sameconditions as those in Example 9 except for using an Al₂ O₃ powder of 3μm in average grain size as the ceramic powder (material manufacturedfrom Showa Denko Co. was pulverized for adjusting the grain size),carbon black used in Example 9 as the non-metallic element powder and aTi powder of 3 μm in average grain size as the metal element powder andsetting the volume ratio of the mixture of carbon black and Ti powder tothe Al₂ O₃ powder to 40% (sintered weight 1.5 g).

The resultant sintered material was extremely dense and it could beconfirmed from X-ray diffractiometry that it was 2-phase Al₂ O₃, TiCsintered composite material. The crystal grain size in each of thephases was 3 μm and 1-3 μm respectively and it could be confirmed thatextremely preferred inter-granular bonding was attained. The resultantsintered material had micro Vickers hardness of 1700 to 1800 kg/mm²under the load of 200 g.

EXAMPLE 11

A composite ceramic was manufactured under the same conditions as thosein Example 9 except for using a ZrO₂ powder of 0.02 μm in average grainsize as the ceramic powder (tetragonal system powder stabilized with1.94 mol % Y₂ O₃) boron of 0.5 μm in average grain size as thenon-metallic element powder and a Ti powder of 3 μm as the metal powderand setting the volume ratio of the mixture of the boron powder and theTi powder to the ZrO₂ powder as 30% (sintered weight of 1.8 g).

The resultant sintered material comprised dense and fine crystal grainsas in Example 10 and the resultant sintered material had a fracturetoughness K_(IC) of 8 MNm^(-3/2).

EXAMPLE 12

A powder mixture to be sintered was obtained by using a Bi-Si₃ N₄ powderof 0.65 μm in average grain size and α-phase ratio of 1% as the ceramicpowder (manufactured by Hermann C. Starck Co.), carbon black andamorphous silicon powders used in Example 9 as the non-metallic elementpowder and setting the volume ratio of the mixture of silicon and carbonblack to the β-Si₃ N₄ powder to 40%. 1.6 g of specimen was sampled fromthe powder mixture to obtain a disc-like mixed lump to be sintered of12.8 mm diameter. The mixed lump to be sintered was charged in the highpressure device shown in FIG. 3 and a sintered composite ceramicmaterial was manufactured under the same conditions as those in Example5 except for changing the heat generation amount of the Si-thermitcomposition and the thermit composition comprising a mixture of ironoxide and Al powder to 33.8 Kcal.

The resultant composite sintered material comprised a mixture of fineβ-Si₃ N₄ and β-SiC and had firm inter-granular bonding. When the microVickers hardness (under 200 g load) of the sintered material wasmeasured, a value of 2100 kg/mm² was obtained. As a result of X-raydiffractiometry, unreacted product of silicon and carbon was notconfirmed.

EXAMPLE 13

For reducing the porosity and improving the characteristics of the TiB₂and NiB 2-phase sintered composite material obtained in Example 5, a HIPtreatment was applied under an Ar atmosphere at 2000 atm. The HIPtemperature was set to 800° C. and the time of applying the pressure wasset to 30 min. The bulk density was increased by about 10% after the HIPtreatment. The heat conductivity at 800° C. was increased to 0.50 W/cm°C. Similar effect of the treatment was confirmed also in the hot presstreatment. In the hot press treatment, the bulk density was increased byseveral % by the application of a pressure of 200 atm at 900° C. for 20min in vacuum and improvement for the heat conductivity by about 10% wasalso recognized.

Also for the TiB₂ -TiB sintered material shown in Example 7, fracturetoughness K_(IC) of the sintered material was increased about from 3 to5 MNm^(-2/3), and the change of the fracture mode from theintra-granular fracture mode to the grain boundary fracture mode wasalso recognized as the mode of the fracture. However, the treatment withthe HIP temperature in excess of 1700° C. or the HIP time in excess of60 min is not preferred since it remarkably changes the fine texture ofthe sintered material or lacks in an economical advantage. As shown inthis example, the HP or HIP treatment for the reduction of porosity, andthe improvement of the reliability and the performance of the sinteredmaterial without changing the fine texture, phase constitution or thelike of the sintered composite material, applied subsequent to themanufacture of the sintered composite material is extremely effectivemeans in the manufacture of the high performance composite ceramic.

The foregoing explanations are merely for the demonstration of suitableexamples in the present invention and the scope of the present inventionis not limited to them. Those skilled in the art can easily thought outfurther various modification examples regarding this invention withoutdeparting the scope of the present invention.

We claim:
 1. A method of manufacturing a sintered ceramic material whichcomprises a step of previously applying a pre-heating to a powder to besintered at a predetermined temperature for a predetermined period oftime and a step of applying heat-sintering to the powder to be sinteredapplied with said pre-heating by the heat generated in a thermitreaction under pressure.
 2. A method as defined in claim 1, wherein thepre-heating is conducted by vacuum heating or an atmospheric heating inan inert atmosphere, a reducing atmosphere or an oxidative atmosphere.3. A method as defined in claim 2, wherein the pre-heating is conductedat a temperature higher than 500° C.
 4. A method as defined in claim 1,wherein a preliminary molding is applied previously preceding to thepre-heating.
 5. A method as defined in claim 4, wherein the preliminarymolding is conducted by a cold isostatic molding or hot isostaticmolding method.
 6. A method as defined in claim 1, wherein theheat-sintering is conducted under a pressure of higher than 200 atm. 7.A method as defined in claim 1, wherein the sintering step by the heatgenerated in the thermit reaction is further followed by post-heating ata predetermined temperature for a predetermined period of time for theimprovement of the reliability and the performance of the sinteredmaterial without changing the fine texture of the resultant sinteredceramic material and the homogenity of the fine texture.
 8. A method asdefined in claim 7, wherein the post-heating is conducted by vacuumheating or an atmospheric heating is an inert atmosphere, a reducingatmosphere or an oxidative atmosphere.
 9. A method as defined in claim7, wherein the post-heating is conducted under pressure of 1 to 200 atmto the sintered material.
 10. A method as defined in claim 8 or 9,wherein the post-heating is conducted by hot isostatic press, hot pressor heat treatment under a normal temperature.
 11. A method as defined inclaim 1, wherein the powder to be sintered comprises one or more ofceramic powder or a mixture of a ceramic powder and a non-metallicand/or metal powder.
 12. A method of manufacturing a sintered compositeceramic material, wherein a mixture comprising:(A) at least one ceramicpowder, (B) at least one of non-metallic powders selected from the groupconsisting of carbon, boron and silicon and (C) a metal powder and/or anon-metallic powder different from (B) described above is heat-sinteredby the heat generated in a thermit reaction under pressure.
 13. A methodas defined in claim 12, wherein the thermit composition for heating thepowder mixture to be sintered is a mixture comprising a copper oxidepowder and an aluminum powder.
 14. A method as defined in claim 12,wherein the thermit composition for heating the powder mixture to besintered comprises a combination of a mixture of a copper oxide powderand an aluminum powder and a mixture of an iron oxide powder and an Sipowder.
 15. A method as defined in claim 12, wherein the heat-sinteringis conducted under a pressure of higher than 200 atm.
 16. A method asdefined in claim 12, wherein the sintering step for the compositeceramic is followed by post-heating at a predetermined temperature for apredetermined period of time for reducing the porosity and improving thereliability and the performance without changing the fine texture, phasecomposition, structure or the like of the resultant sintered compositematerial.
 17. A method as defined in claim 16, wherein the post-heatingis conducted by vacuum heating or an atmospheric heating in an inertatmosphere, a reducing atmosphere or an oxidative atmosphere.
 18. Amethod as defined in claim 16, wherein the post-heating is conducted bya hot isostatic press, hot press or a heat heat treatment under a normalpressure.
 19. A method as defined in claim 16, wherein a hot isostaticpress or hot press treatment is applied for 5 to 60 min at a temperatureof 500° to 1700° C. and under a pressure within a range of 200 to 2000atm as the post-treatment.
 20. A method as defined in claim 12, whereinthe powder mixture to be sintered is previously applied with pre-heatingat a predetermined temperature for a predetermined period of timepreceding to the sintering step.
 21. A method as defined in claim 20,wherein the pre-heating is conducted by vacuum heating or atmosphericheating in an inert atmosphere, a reducing atmosphere or an oxidativeatmosphere.
 22. A method as defined in claim 21, wherein the pre-heatingis conducted at a temperature higher than 500° C.